Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753380AbbBWW4W (ORCPT ); Mon, 23 Feb 2015 17:56:22 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:47274 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752413AbbBWW4U (ORCPT ); Mon, 23 Feb 2015 17:56:20 -0500 Date: Mon, 23 Feb 2015 14:56:19 -0800 From: Andrew Morton To: Davidlohr Bueso Cc: Vlastimil Babka , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Andrea Arcangeli , "Kirill A. Shutemov" , Rik van Riel , Mel Gorman , Michal Hocko , Ebru Akagunduz , Alex Thorlton , David Rientjes , Peter Zijlstra , Ingo Molnar Subject: Re: [RFC 0/6] the big khugepaged redesign Message-Id: <20150223145619.64f3a225b914034a17d4f520@linux-foundation.org> In-Reply-To: <1424731603.6539.51.camel@stgolabs.net> References: <1424696322-21952-1-git-send-email-vbabka@suse.cz> <1424731603.6539.51.camel@stgolabs.net> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1914 Lines: 35 On Mon, 23 Feb 2015 14:46:43 -0800 Davidlohr Bueso wrote: > On Mon, 2015-02-23 at 13:58 +0100, Vlastimil Babka wrote: > > Recently, there was concern expressed (e.g. [1]) whether the quite aggressive > > THP allocation attempts on page faults are a good performance trade-off. > > > > - THP allocations add to page fault latency, as high-order allocations are > > notoriously expensive. Page allocation slowpath now does extra checks for > > GFP_TRANSHUGE && !PF_KTHREAD to avoid the more expensive synchronous > > compaction for user page faults. But even async compaction can be expensive. > > - During the first page fault in a 2MB range we cannot predict how much of the > > range will be actually accessed - we can theoretically waste as much as 511 > > worth of pages [2]. Or, the pages in the range might be accessed from CPUs > > from different NUMA nodes and while base pages could be all local, THP could > > be remote to all but one CPU. The cost of remote accesses due to this false > > sharing would be higher than any savings on the TLB. > > - The interaction with memcg are also problematic [1]. > > > > Now I don't have any hard data to show how big these problems are, and I > > expect we will discuss this on LSF/MM (and hope somebody has such data [3]). > > But it's certain that e.g. SAP recommends to disable THPs [4] for their apps > > for performance reasons. > > There are plenty of examples of this, ie for Oracle: > > https://blogs.oracle.com/linux/entry/performance_issues_with_transparent_huge hm, five months ago and I don't recall seeing any followup to this. Does anyone know what's happening? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/